2.3 Using Layered Models

3.1 Using Layered Models
Protocols for special-purpose tasks are grouped into well defined layers. Some of benefits of using standard layered models are:

	Assists in designing, maintaining, and updating protocols.
	Software and hardware that meet the standard model will be able to communicate with
	software and hardware that also meet the model.
	Given users alternative choices to use software and hardware produced by different companies and
	manufacturer from different countries.
	The protocols are defined at each stage and so errors can be handled.

  3.1.1 TCP/IP Model

TCP/IP stands for the Transport Control Protocol / Internet Protocol suite.

	The TCP/IP protocol architecture was developed by research funded the defense advanced research
	project agency. It consists of many protocols that have been issued as Internet standards by the
	Internet Activity Board (http:// www.iab.org).

2.3 Using Layered Models

It consists of four layers:

		Layer 4, the application Layer, consists of application programs and user interfaces.
		Layer 3, the transport layer, supports communication between devices and performs
		error correction.
		Layer 2, the Internet layer, finds the best path through the network.
		Layer 1, the network access, controls hardware devices and transmission medium.

2.3 Using Layered Models

The main components of TCP/IP are:

		TCP - is responsible for verifying the correct delivery of data from client to server. Data can be lost in
		the intermediate network. TCP adds extra information (control information or header) to detect errors
		or lost data and to trigger retransmission until the data is correctly and completely received.
		IP - is responsible for moving packet of data from device to device. IP forwards each packet
		based on a four byte destination address (the IP number). The Internet authorities assign ranges
		of numbers to different organizations.

2.3 Using Layered Models

The process of adding header to the data as it passes through the layered model is called encapsulation. Decapsulation is the process of removing the extra information (header) and sending only the original application data up to the destination application layer. The generic term for data at each level is protocol data unit (PDU). In encapsulation, PDU is also called data encapsulation.

2.3 Using Layered Models

3.2 The OSI Model

The Open Systems Interconnection (OSI) model was developed, in 1984, by ISO as a framework to develop protocol standards and as network protocol architecture. The OSI model divides network communication into seven layers.

		Layer 7, the application layer, provides the interface between user communication applications
		and the underlying network over which user messages are transmitted.
		Layer 6, the presentation layer, defines the format used to exchange data among communicating
		devices. It also responsible for encrypting, and compressing data.
		Layer 5, the session layer, provides applications on different devices to open, maintain, and close a
		connection called a session.
		Layer 4, the transport layer, provides a reliable mechanism to exchange data between two
		applications in different devices with ensuring that the sent data units are delivered in order and
		without loss, duplication or error in data.

2.3 Using Layered Models

		Layer 3, the network layer, is used to route data across configured network devices. This layer
		uses many technologies to enable the data to be transmitted which are known as switching and
		routing. These technologies create logical (known as virtual circuits) paths for transmitting
		data across a network from a device to device transmission.
		Layer 2, the data-link layer, prepares the data so that the receiver device can understand
		it. This layer creates and addresses frames for host-to-host delivery on the local LANs and
		between WAN devices.
		Layer 1, the physical layer, transmits binary data (raw bit stream) over the transmission
		medium between the source and destination. This layer is completely hardware-oriented
		and deals with all aspects of establishing and maintaining a transmission medium between
		communicating devices.

2.3 Using Layered Models

The three upper layers (application, presentation and session) are mainly concerned with services to the applications, while the four lower layers (transport, network, data link and physical) are concerned with data flow from device to device over the network. The data encapsulation in OSI model is shown in the following figure.

2.3 Using Layered Models

3.3 Comparison between OSI and TCP/IP Models

The main differences between the two models are as follows:

		The TCP/IP model is faster and more practical than the OSI model in network communication.
		TCP/IP Protocols are considered to be standards around which the internet has developed.
		The OSI model however is a "generic, protocol independent standard"
		TCP/IP combines the presentation and session layer issues into its application layer.
		TCP/IP combines the OSI data link and physical layers into the network access layer.